Thermodynamic Study of Ce⁴⁺ /Ce³⁺ Redox Reaction in Aqueous Solutions at Elevated Temperatures: 1. Reduction Potential and Hydrolysis Equilibria of Ce⁴⁺ in HCIO₄ Solutions

Abstract

The redox potential (E) of the couple Ce 4+ /Ce 3 + has been determined up to 368 K by means of cyclic voltammetric measurement in aqueous HC10 4 solutions with c HC104 decreasing from 7.45 to 0.023 mol kg" 1 . A constant potential of (1.741 V) 298 K , resp. (1.836 V) 368K , indicating the existence of pure unhydrolysed Ce 4+ was obtained at c Hcl04 > 6.05 m. At lower HC10 4 concentration, the potential as a function of the HC10 4 molality, as well as of the pH shows 4 further distinct steps. At 298 K, for instance, the potential became nearly constant at pH values of 0.103, 0.735,1.115, after which it drastically decreased, respectively at 1.679, just before the precipitation of Ce(OH) 4 occurred. The curves indicate obviously the stepwise formation of the Ce(IV) mono-, di-, tri-and tetrahydroxo complexes. The slope of the curves E vs. pH increased gradually with increasing temperature. AS and AH of the redox reaction were determined as functions of T at the different HC10 4 concentrations. AS is positive at c HC104 > 1.85 m and turns to be negative at lower concentrations. AH is negative at all HC10 4 concentrations studied. The cumulative formation constants ß, of the Ce(IV) hydroxo complexes and the corresponding hydrolysis constants (K h ); were calculated. An unusual decrease of ß t with increasing temperature has been discussed.

New kinetic and mechanistic findings in the oxidation of hydroxylamine by Cerium(IV) in perchloric acid media

“…When Ce(IV) is employed in sulfuric acid or nitric acid, a number of complexes between Ce(IV) and sulfate or nitrate are formed depending on the concentrations of sulfate or nitrate in the reaction media. However, there are no evidence for complex formation between Ce(IV) and perchlorate . The kinetic results obtained in this work are very different from those reported previously in the sulfuric acid and nitric acid: (a) the reaction in the perchloric media is much faster.…”

“…The three Ce(IV) species are correlated by two stepwise hydrolysis equilibria, which are described by Equations and : Values of K h 1 = 6.40 and K h 2 = 0.12 have been reported in perchloric acid media at 24.5°C, μ = 1.0 M; furthermore, these values are not influenced by the ionic strength change . On the contrary, Ce 3+ is not hydrolyzed in acidic media when pH < 3 …”

Fluid inclusion and microfabric studies on Zechstein carbonates (Ca2) and related fracture mineralizations – New insights on gas migration in the Lower Saxony Basin (Germany)

“…30 Positive Eu anomalies usually occur when Eu is thermochemically reduced under high-temperature conditions (> 250°C) (Bilal and Müller, 1992). Since no Eu anomaly was detected either in the primary precipitate (Fluorite A) or in the recrystallized fluorite (Fluorite B) temperatures during precipitation and remobilization did not exceed 250°C, which correlates with measured homogenisation temperatures deduced from primary fluid inclusions.…”

Aqueous chemistry of Ce(iv): estimations using actinide analogues

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